Go native-only: remove the Python POC, move the Swift app to the top level

Adds --check (compile every effect without camera or window), EFFECTS.md
(the contract for writing effects) and AGENTS.md. The POC stays available
at the 'poc' tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Sources/MotionLab/Panel.swift

@@ -0,0 +1,92 @@
+import AppKit
+
+private final class TopAlignedView: NSView {
+    override var isFlipped: Bool { true }  // so the scroll view starts at the top
+}
+
+/// Side panel: every effect with a checkbox, and the sliders of the ones that are switched on.
+final class Panel: NSObject {
+    static let width: CGFloat = 230
+
+    let view = NSScrollView()
+    private let stack = NSStackView()
+    private let statusLabel = NSTextField(labelWithString: "")
+    private let renderer: Renderer
+
+    init(renderer: Renderer) {
+        self.renderer = renderer
+        super.init()
+        stack.orientation = .vertical
+        stack.alignment = .leading
+        stack.spacing = 4
+        stack.translatesAutoresizingMaskIntoConstraints = false
+
+        let document = TopAlignedView()
+        document.translatesAutoresizingMaskIntoConstraints = false
+        document.addSubview(stack)
+        view.documentView = document
+        view.hasVerticalScroller = true
+        view.drawsBackground = false
+        NSLayoutConstraint.activate([
+            view.widthAnchor.constraint(equalToConstant: Panel.width),
+            document.widthAnchor.constraint(equalTo: view.contentView.widthAnchor),
+            stack.topAnchor.constraint(equalTo: document.topAnchor, constant: 14),
+            stack.leadingAnchor.constraint(equalTo: document.leadingAnchor, constant: 14),
+            stack.trailingAnchor.constraint(equalTo: document.trailingAnchor, constant: -14),
+            stack.bottomAnchor.constraint(equalTo: document.bottomAnchor, constant: -14),
+        ])
+        statusLabel.font = .boldSystemFont(ofSize: 11)
+        statusLabel.textColor = .systemRed
+        rebuild()
+    }
+
+    private func label(_ text: String, dim: Bool = true) -> NSTextField {
+        let field = NSTextField(labelWithString: text)
+        field.font = .systemFont(ofSize: 11)
+        field.textColor = dim ? .secondaryLabelColor : .labelColor
+        return field
+    }
+
+    func rebuild() {
+        stack.arrangedSubviews.forEach { $0.removeFromSuperview() }
+        statusLabel.stringValue = renderer.status
+        stack.addArrangedSubview(statusLabel)
+
+        for (layerIndex, layer) in renderer.layers.enumerated() {
+            let title = layer.error == nil ? layer.name : "\(layer.name)  (shader error)"
+            let checkbox = NSButton(checkboxWithTitle: title, target: self, action: #selector(toggled(_:)))
+            checkbox.state = layer.enabled ? .on : .off
+            checkbox.tag = layerIndex
+            checkbox.font = .systemFont(ofSize: 13, weight: .medium)
+            stack.addArrangedSubview(checkbox)
+            guard layer.enabled else { continue }
+
+            for (paramIndex, param) in layer.params.enumerated() {
+                let slider = NSSlider(value: Double(param.value), minValue: Double(param.min), maxValue: Double(param.max),
+                                      target: self, action: #selector(sliderMoved(_:)))
+                slider.tag = layerIndex * maxParams + paramIndex
+                slider.controlSize = .small
+                stack.addArrangedSubview(label(param.name))
+                stack.addArrangedSubview(slider)
+                slider.widthAnchor.constraint(equalTo: stack.widthAnchor).isActive = true
+            }
+            stack.setCustomSpacing(12, after: stack.arrangedSubviews.last!)
+        }
+        if renderer.layers.isEmpty { stack.addArrangedSubview(label("no .metal files found")) }
+
+        stack.setCustomSpacing(16, after: stack.arrangedSubviews.last!)
+        stack.addArrangedSubview(label("s  screenshot\nr  record\nq  quit"))
+    }
+
+    func refreshStatus() {
+        statusLabel.stringValue = renderer.status
+    }
+
+    @objc private func toggled(_ checkbox: NSButton) {
+        renderer.setEnabled(checkbox.tag, checkbox.state == .on)
+    }
+
+    @objc private func sliderMoved(_ slider: NSSlider) {
+        renderer.setParam(layer: slider.tag / maxParams, param: slider.tag % maxParams, value: Float(slider.doubleValue))
+    }
+}